a) Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to techniques of improving the display quality of a liquid crystal display device by using optical compensation components.
b) Description of the Related Art
A liquid crystal display device having a homeotropic orientation liquid crystal cell disposed between two polarizers shows sharp threshold characteristics in an ECB (electrically controlled birefringence) mode. A high duty is therefore possible by matrix time divisional drive. However, light incidence in an oblique direction relative to the display screen of a liquid crystal display device has parasitic birefringence. Therefore, there is light transmission even without voltage application, and the contrast lowers considerably as compared to light incidence in a perpendicular direction relative to the display screen.
In order to solve this problem, optical compensation has been proposed by combining an optical compensation plate with an ECB mode liquid crystal cell as shown in FIG. 4. Reference numeral 10 represents an ECB mode liquid crystal cell which is made of nematic liquid crystal having a positive refractive index anisotropy and a negative dielectric anisotropy and being disposed generally vertical to a glass substrate surface having electrodes. Two orthogonal Nicol configuration linear polarizers 12 and 13 sandwich the ECB mode liquid crystal cell 10. When a voltage is not applied, the cell appears black, and when a voltage is applied, the cell enters a light transmission state and appears white.
An optical compensation plate 11 is inserted between the liquid crystal cell 10 and the linear polarizer 13, the optical compensation plate 11 having a negative refractive index ellipsoid indicated at 15 in FIG. 4. A combination of the positive anisotropic refractive index ellipsoid 14 of the liquid crystal cell 10 and the negative refractive index ellipsoid 15 of the optical compensation plate 11 produces optical isotropy so that the optical compensation plate 11 functions as a view angle compensator. In FIG. 4, n. represents an extraordinary ray refractive index, and no represents an ordinary ray refractive index.
Optical compensation plates having a property described above have been manufactured by the following methods.
(1) A mixture of inorganic layer compound and polymer or the like is coated on a film (JP-A-5-196819, JP-A-6-82777).
(2) Discotic liquid crystal is coated on a film.
(3) A polycarbonate film is pulled or drawn in two directions.
(4) A thermosetting film is held between two glass plates and applied with heat and pressure (JP-B-7-69536).
The films formed by the above methods are all negative uniaxial.
In an ideal case of the optical compensation plate shown in FIG. 4, a combination of the positive anisotropic refractive index ellipsoid 14 of the liquid crystal cell 10 and the negative refractive index ellipsoid 15 of the optical compensation plate 11 produces optical isotropy, and the optical compensation plate 11 functions as a view angle compensator. In this case, the negative refractive index ellipsoid 15 of the optical compensation plate 11 is required to be a negative uniaxial or biaxial refractive index ellipsoid. Optical compensation by a positive refractive index ellipsoid is impossible.
The structure of a liquid crystal display device using a conventional optical compensation plate made of films formed by either of the methods (1) and (2) will be described with reference to FIG. 5 which is a cross sectional view of a lamination structure of a liquid crystal cell, a compensation film, and a polarizer. On a liquid crystal cell 10, an optical compensation film 17 is adhered with a binder layer 16. On the optical compensation film 17, a polarizer 18 is adhered with a binder layer 19.
As a base for the polarizer 18 and optical compensation film 17, TAC (triacetate cellulose) is generally used because of a good balance between cost and performance. The polarizer 18 has a structure of a polarizer layer 18a sandwiched between a pair of TAC films 18b and 18c. The optical compensation film 17 is formed by coating a negative uniaxial film layer 17b on the surface of a base TAC film 17a.
The material of the negative uniaxial film layer 17b is an inorganic layer compound and an organic binder such as polyvinyl alcohol (PVA). Therefore, a birefringence dependency of this film 17b upon wavelength is determined by the inorganic layer compound. Generally, the refractive index dependency of an inorganic compound upon wavelength is almost constant over wavelengths of visible light rays. However, it is known that since liquid crystal is an organic compound including an aromatic compound, the refractive index greatly depends upon wavelength. Generally, both the ordinary and extraordinary ray refractive indices tend to become larger on the shorter wavelength side.
Therefore, a combination of a liquid crystal cell and a compensation plate made of an inorganic layer compound cannot realize an optical compensation uniformly over the whole range of wavelengths of visible rays. In the example described above, if the birefringence near 550 nm is optimized, a yellowish image appears on the display screen when viewed obliquely. In particular in a color display, color tone of blue becomes near achromatic color.
If a TAC film is exposed for a long time in an atmosphere at 60.degree. C. or higher or in high temperature and humidity environments, the film shrinks in arrow directions as shown in FIGS. 6A and 6B and optical uniaxes are induced in the shrinking directions by a shrinking stress, independently from the optical axis inherent to the TAC film. FIG. 6A is a plan view of the TAC film, and FIG. 6B is a cross sectional view along a dotted chain line of FIG. 6A, in a liquid crystal display device as shown in FIG. 4. Optical uniaxes caused by shrinking have different directions at different areas because of different shrinking stresses in various in-plane areas, although they are influenced by the shape and size of the film. Therefore, even if the polarizer 18 is adhered in an optimum direction, there is always an in-plane area where the direction of a uniaxis is shifted from a predetermined angle to the transmission/absorption axis of the polarizer. The effects of the optical compensation plate are therefore lost, and the induced uniaxis generates birefringence and a white spot (leakage light) is formed on the display screen.